Stability of the polar {111} NaCl crystal face

We present a surface x-ray diffraction determination of the {111} NaCl-liquid interface structure. Using ultrathin water or formamide liquid layers we ascertained that the crystal surface is smooth at an atomic level and is not reconstructed. Our results reveal surprisingly small differences in surface structure between the two cases, which nevertheless lead to dramatic differences in crystal morphology. We determined that the rocksalt {111} surface is Na(+) terminated for both environmental conditions. A quarter to half a monolayer of laterally disordered Cl(-) ions is located on top of a fully ordered Na(+) crystal surface with occupancy 0.75-1.0. This means that the polar surface is stabilized through the formation of an electrochemical double layer.